Exoplanet science has literally opened new worlds to study, with planets populating the galaxy unlike anything in our small solar system. Hot Jupiters whip around their stars in just days, burning at thousands of degrees. Super Earths—rocky planets that are more massive than our own—offer intriguing targets to study for signs of life.
One planet, called K2-18b, sits approximately 110 light-years away from Earth. It’s larger than our planet, about 8.6 times the mass, and bigger in size at about 2.7 times the radius. These types of planets are commonly referred to as mini-Neptunes, thought to have rocky or icy cores surrounded by expansive atmospheres, and in recent years, scientists have found that they are extremely common across the galaxy.
K2-18b is enveloped by a large atmosphere of mostly hydrogen, and new research, using observations from the Hubble Space Telescope, reveals that K2-18b’s atmosphere also contains water molecules in the form of vapor and possibly clouds that contain liquid droplets of H2O. The finding is the first detection of water on an exoplanet in the habitable zone, where the water molecules could be liquid, making it an exciting step toward finding a planet that could support life as we know it.
“This planet is definitely smaller than any other planet water has been detected in, and it is also colder,” says Laura Schaefer, an assistant professor of geological sciences at Stanford who studies planetary atmospheres and was not involved in the new research.
Astronomers don’t know exactly what K2-18b looks like, but the modeled pressures and temperatures of the planet suggest that clouds may form in its skies, possibly with liquid rain. “[The planet] could be a white-ish planet that looks like water clouds from the top,” says Björn Benneke, a professor of astronomy at the University of Montreal and lead author of a new study describing K2-18b submitted to the Astronomical Journal and posted to arXiv, an online database of draft research papers that have yet to publish in a peer-reviewed journal. A study published today in Nature Astronomy by a group of researchers from University College London also uses the Hubble data to identify water vapor on K2-18b.
K2-18b orbits a red dwarf, which is a relatively small and cool star. But K2-18b is much closer to its star than Earth is to the sun, completing an orbit in just 33 days, so both planets receive about the same amount of energy.
“K2-18b is very different from anything we know,” says Sara Seager, a professor of physics and planetary science at MIT not involved in the research, in an email. “There are no solar system counterparts.”
The watery planet is not particularly Earth-like, and it’s doubtful that K2-18b has a rocky surface like that of our planet, Seager and Shaefer say. The density measurements of K2-18b suggest the atmosphere is mostly hydrogen, surrounding a rocky and icy core. Within this hydrogen, however, is the distinct mark of H2O.
In 2016 and 2017, Benneke led a team that used Hubble to measure light from K2-18b’s star that had passed through the atmosphere of the planet. Specifically, the Wide Field Camera 3 observed the light at wavelengths around 1.4 micrometers to look for the chemical signature of water, which appears as dark lines missing in the light’s spectrum. Additional observations from the Spitzer Space Telescope and the Kepler space telescope, which was used to discover K2-18b in 2015, helped fill out the spectrum even more to reveal the presence of water.
“Every molecule has a unique signature,” Benneke says, referring to the lines that different molecules create in a spectrum of light. “Water has really strong absorption bands, especially in that wavelength they are looking at,” Schaefer adds.
In Earth’s atmosphere, water vapor’s penchant for absorbing light limits the effectiveness of ground-based telescopes. But that same absorption quality makes water relatively easy to pick out in another planet’s atmosphere, compared to other molecules such as carbon dioxide.
Based on models of K2-18b, Benneke’s research group thinks the planet likely has a cloud deck hovering somewhere in the atmosphere between 1 bar of pressure—about the pressure at sea level on Earth—and .01 bar, which roughly corresponds with the pressure in Earth’s atmosphere 100,000 feet above the surface. From this cloud deck, rain droplets could form and fall into the planet.
“The temperature of the atmosphere increases the deeper you go,” Benneke says. “So when these water droplets condense … they drop out and fall to deeper and deeper layers, and these layers are warmer. So the water will, on the way, evaporate again.”
This cycle of rain and evaporation is not all that different from processes on Earth. Drops of rain can fall over hot deserts and evaporate back into water vapor before they hit the ground, for example. “In some ways, it’s just like on Earth, except [K2-12b] has no surface,” Benneke says.
The hydrological cycle of rain and evaporation on K2-18b is “a solid but still speculative” idea, Seager says. Whether clouds form in the atmosphere where liquid water can exist is based on planetary models.
“Models are really essential for the planning, but of course in all the observations we have to be willing to accept really unexpected and new things,” Schaefer says.
Hubble’s Wide Field Camera 3 happens to cover the wavelength range that includes water’s absorption lines, making it possible for us to spot the molecules. But for smaller, rocky planets, astronomers will need more powerful telescopes. The James Webb Space Telescope, slated to launch in 2021, will be able to not only confirm the presence of water on K2-18b, but also measure other molecules and characterize the atmospheres of smaller planets more similar to Earth.
This type of observation—measuring the light that has passed through a planetary atmosphere to pick out specific chemical signatures—remains on the very forefront of telescope capability.
“It’s definitely pushing the limits of what’s been done before,” Schaefer says.
Up to this point, the technique has primarily been used to study very large gas planets that are relatively close by. But the detection of water on K2-18b proves that it is possible to learn what the atmospheres of smaller planets are made of, taking scientists one step closer to discovering a world like our own.
Editor’s Note, September 11, 2019, 1:30 p.m. EDT: This story has been updated to include an additional study about K2-18b published in Nature Astronomy.